The invention relates to a process of manufacturing nanocrystalline metal hydrides.
It is known that hydrogen storage devices, known as hydride storage devices, can be formed on the basis of reversible metal hydrides. This involves charging the storage device under release of heat, wherein hydrogen is bonded by chemisorption and is discharged again by the application of heat. Thus hydrogen storage devices could form outstandingly good energy storage devices for mobile and/or stationary applications, i.e. they should provide in the future considerable storage potential because no harmful emissions are generated during the discharge of the hydrogen storage device.
What are known as nanocrystalline hydrides are very suitable for this kind of hydride storage devices. These hydrides are characterized by rapid hydrogen assimilation and release kinetics. However, until now, their manufacture has been very complicated and expensive. Up to now, nanocrystalline alloys were manufactured firstly by high-energy grinding from elemental components or pre-alloys, with the grinding durations sometimes being very long. In a subsequent process step, these nanocrystalline alloys were subjected, where required, to a multi-stage heat treatment under a high hydrogen pressure to be hydrogenated in this manner. Furthermore, for many alloys, multiple charging and discharging with hydrogen is necessary in order to achieve full storage capacity.
Alternatively, attempts have been made to synthesize the corresponding hydrides by milling in an atmosphere of hydrogen or by purely chemical means. However, it was observed that the yield of the desired hydrides is relatively small and additional unwanted phases may sometimes appear.
Furthermore, certain phases are not even obtainable with these conventional methods.
It is the object of the present invention to provide a process of manufacturing metal hydrides, with which however stable and metastable hydrides or hydrides of metastable alloys can be obtained, particularly hydrides with a very high yield of up to 100%. The process should further be capable of being carried out under comparatively simply manageable conditions and should require a comparatively small input of energy.
In a process for manufacturing nanocrystalline metal hydrides, a metal hydrides of a first kind is subjected to a mechanical milling process with at least one elemental metal or at least one additional metal hydride to produce an alloy hydride.
The advantage of the process according to the invention lies essentially in the fact that the manufacture of stable and metastable hydrides or hydrides of metastable alloys is made possible in a comparatively simple way with a high yield of up at 100%. The disadvantages that occur in the processes known in the state of the art for manufacturing hydride storage devices are avoided. In addition, the process according to the invention permits the manufacture of hydrides that could not be manufactured at all using known processes.
Depending on the hydrides used to manufacture nanocrystalline metal hydrides, the milling process for the mixture of elemental metal hydride, metal or several additional metal hydrides is preferably carried out for a predetermined period, preferably in the range from 20 to 200 hours.
In principle, however the milling procedure period is dependent on the design of the milling equipment used, so that the specified and preferred milling times may not be reached or may be exceeded. However, in general, it can be said that the milling times according to the invention are significantly shorter than those employed during milling without the use of hydrides.
Milling under an inert gas atmosphere has been found to be advantageous. As already mentioned above, hydrides, for example magnesium-iron hydrides, were hitherto manufactured by annealing at high temperature under a high pressure of hydrogen. Remaining with this example, attempts were made to mill magnesium and iron in a hydrogen atmosphere, !but this did not lead to the generation of the desired magnesium-iron hydride. However, according to the invention, it is possible, by milling magnesium hydride and iron in a particular molar ratio under an inert gas atmosphere, to produce a hydrogen-enriched hydride directly at the end of the milling process, which has proved very successful especially when using argon as the inert gas.
Particularly good results were achieved with the process, when the first elemental metal hydride consisted of metals of the Ist or IInd main group of the period system. The metals are preferably Li, Na, K, Mg, Ca, Sc, Y, Ti, V, Ny, or La, with the elements preferably being Fe, Co, Nb, Cu, Zn, Al and Si. Especially good process results were also achieved when the elemental metal consisted of elements of the VIIIth sub-group of the periodic system of elements.
Preferably, the second metal hydride consists of a mixture of elements of the Ist and IIIrd main group of the periodic system of elements. Carrying out the process in such a way provides for very good results in the desired sense.
Basically, the process can also be carried out if the metal hydrides and/or the metal are not present in the form of powder at the beginning of the milling procedure. It is especially advantageous to first convert the metal hydride and/or the metal into powder form and then to subject the powdered metal hydride and/or the metal to the milling process according to the invention because the process can then be operated efficiently and consequently with an extremely high yield.
The invention will now be described in detail on the basis of several examples with reference to the following diagrammatic illustrations.